Landfill biofiltration system and methods for removal of gas-phase pollutants and contaminants

ABSTRACT

Biochemical decomposition of undesirable gaseous contaminants, including nitrogen oxides, VOC&#39;s, carbon monoxide and sulfur oxides, and malodorous contaminants, is achieved by passing a gas stream through a managed landfill providing microbiological activity capable of degrading the contaminants. Gases suitable for treatment include fuel combustor exhaust, landfill gases, putrescent gases and the like. The landfill functions as a biological reactor (bioreactor), where water is added if or as necessary to achieve concentrations between about 20% and about 65% by weight, and desired microbial contaminant abatement action. By a permeation of the polluted gas through the landfill, there is a consumption of polluting gases by microorganisms present in the landfill. The process enables increased combustion of fuels, such as landfill gases, whose energy values are currently wasted or are not available due to emissions problems. The excess oxygen normally present in exhaust can advantageously result in additional oxidative waste consumption by microorganisms, yielding additional “air space” that is an economic bonus in extending landfill life and/or lessening landfill use.

FIELD OF THE INVENTION

The field of the invention is the use of waste in landfills, and similarlarge tonnage waste masses such as large waste piles in dumps, incombination with appropriate landfill or waste dump managementtechniques, for removal of undesirable biodegradable noxious gases fromemitted gas streams.

BACKGROUND OF THE INVENTION

The creation and emission of vapor-phase gas pollutants can be due tomany human activities. For example, such pollutants can result from fuelcombustion for energy, including fossil fuels, landfill gases and woodgasifier product gas. Combustion is also used for disposal of wasteproducts which can be burned. Such waste products include gas from solidwaste landfills (landfill gas), waste gas from oil refineries, and solidwaste disposed of in incineration processes. Air streams or process exitgas can also commonly be contaminated with volatile organic compounds(VOC's), e.g. by cooking, fires, emissions of aerosols or theirpropellants, or putrescing wastes.

The combustion of fuels for energy, or burning of materials for disposalresults in contaminants in product gas that include oxides of nitrogen,carbon monoxide, volatile organic compounds, and sulfur oxides. Amongcontaminants of regulatory concern are most often the three major oxidesof nitrogen: nitrous oxide (N₂O), nitric oxide (NO) and nitrogen dioxide(NO₂) and, as well, carbon monoxide. As one example, U.S. emission limitstandards commonly applied to natural gas firing are 0.06 pounds of NOxper one million Btu's and 0.2 pounds of CO (carbon monoxide) per millionBtu's.

Internal combustion (IC, or piston) engines have a number ofattractions, including their ability to use many fuels, such as landfillgas, and ready maintenance. But, despite their numerous advantages, suchengines emit nitrogen oxides in amounts that are about five-fold thoseof other combustion-based mechanical and electrical power options.Depending on fuels and operations, IC Engines can also emit CO andunburned higher (C₂ ⁺ [two-carbon] and up) hydrocarbons. Unburned higherhydrocarbons are then emitted as VOC's, which are regulated local airpollutants. In addition, the combustion of fuels containing sulfur cangive rise to sulfur oxides (SO_(x)). Even as needs for variouscombustion processes increase in the US and worldwide, regulation andrestriction of emissions, particularly NO_(x), and CO, is becoming sostringent as to limit the use of these processes to far less than theirfull potential.

If these contaminant emissions could be reduced to meet regulatorystandards (generally state or district-specific), or even better,eliminated, then the use of some combustion processes would be feasibleboth from economic, environmental and regulatory standpoints. Forexample, solid waste landfills generate combustible gas, which isundesirable if released into the atmosphere because of the presence ofmethane and higher hydrocarbon contaminants. However, if flue or exhaustgases resulting from landfill gas combustion can be appropriatelycleaned, the landfill gas can be more readily used as a renewable fuelthrough its combustion in energy generating plants. This use not onlyabates pollutants, but can also reduce atmospheric emissions of landfillmethane, a gas whose emissions are considered in the aggregate to havemajor adverse climate effects. Such landfill gas fuel use can conserveother fuels as well as reduce emission of fossil CO₂ from lessening useof fossil energy sources. For this reason, landfill gas energy use orabatement has substantial climate benefits. Benefits associated withenergy use are such that the U.S. EPA encourages landfill gas use in itsvery active Landfill Methane Outreach Program (LMOP)

Using landfill gases or other energy sources for electricity generationat multiple smaller-scale sites near population centers, has becomeincreasingly desirable, because it substantially lessens congestion andresultant resistance losses of electric power, especially inlonger-distance electrical transmission lines. Such electricity fuelingover multiple widespread facilities is termed “distributed generation”.As noted, landfill gas combustion, whether for powering internalcombustion engines or for use in other fuel combustors, can increase airpollutant emissions, particularly nitrogen oxides and carbon monoxide.Current emissions standards severely constrain use of landfill gas, infueling such distributed generation. Thus methods for reduction oravoidance of emissions will be of high value.

Simultaneously, even as landfill gas energy use and distributedgeneration is emission constrained, landfill management technology isadvancing concepts showing superior environmental benefits andpotentials—including realization of much-increased landfill gas energy,if energy-related emissions were not barriers (Pacey et al. (1999) TheBioreactor Landfill—An Innovation in Solid Waste Management. Solid WasteAssociation of North America, Silver Spring, Md.)

The present Federal subtitle D landfill regulations evolved with thegoal of keeping waste landfills dry. (This has, in the past decade,become known as the “Dry Entombment” approach). Alternative strategieshave been recognized for some time and are now being rapidly developed.It is being shown that waste decomposition and methane generation can beaccelerated and be better controlled by improving bacterial reactionconditions in “bioreactor” landfills (see, for example, Augenstein etal. (1976); Fuel Gas Recovery from Controlled Landfilling of SolidWastes, Resources and Conservation, 1, 103-117; Barlaz et al. (1990);Methane Production from Municipal Refuse—A Review of EnhancementTechniques and Microbial Dynamics, Critical Reviews in EnvironmentalControl, 19(6):557-584; Stessel et al. (1994) Design Implications of thein-Ground Digester, Proceedings, Air and Waste Management AssociationMeeting Cincinnati, June 19-24; Augenstein et al. (2000) Yolo CountyControlled Landfill Project June, Proceedings, Second InternationalMethane Mitigation Conference, Akademgorodok, Novosibirsk, Siberia,Russia. Proceedings available from US EPA. The bioreactor approach has anumber of benefits as seen by landfill owner/operators and alsoregulators (see Pacey et al. (1999), supra.)

Yet other processes of numerous types are limited by emissions ofpollutant or odorous compounds or both. There are a host of theseprocesses, e.g. animal husbandry, confined animal operations, aerobiccomposting, forcing air through landfills for composting or heatingpurposes, and the like.

The present invention provides methods that reduce undesirablecontaminating gases from combustion or landfill gas fueled generation,or polluted/odorous gases from other sources. A substantial source ofrenewable energy can be freed for economical use, and polluted exhaustgas or other gas streams can be cleaned at a relatively low cost.

Relevant Literature

Patents relating to bioremediation of gaseous pollutants include U.S.Pat. No. 5,503,738 (Apr. 2, 1996), which describes a process forremediating vaporous pollutants by passage through a bioreactorcontaining microorganisms capable of remediating the pollutants. U.S.Pat. No. 5,795,751 (Aug. 18, 1998) discloses a biofilter for reducingconcentrations of gaseous nitrogen oxides in a polluted gas through anorganic filter bed with denitrifying bacteria. U.S. Pat. No. 6,013,512(Jan. 11, 2000) provides an apparatus for scrubbing gaseous emissionswith simultaneous liquid scrubbing and biochemical decomposition ofNO_(x) or VOC or a combination thereof using an aqueous suspension heldin a biomass chamber. U.S. Pat. No. 6,117,672 (Sep. 12, 2000) provides asystem combining biomass filtration, anaerobic digestion, steamabsorption refrigeration and heat exchangers wherein moist wasteconsumes the “nitrogenous oxides” in gases from a fuel combustor.

Other relevant work has been sponsored, for example by the CaliforniaAir Resources Board, in (Hudepohl. N. J., Davidova, Y. du Plessis. C.Schroeder, E. D., and Chang, D. P. Y. 1999 Biofilter Technology for NOxControl. Department of Civil and Environmental Engineering, Universityof California, Davis). This uses a column reactor and nitrifyingorganisms to convert NO to NO₂, thence nitrate.

Patents relating to aerobic landfills include U.S. Pat. No. 5,546,862,relating to a method of landfill mining. U.S. Pat. No. 5,888,022 isdirected to methods of improving aerobic degradation of the solid wastesemplaced in a landfill. U.S. Pat. No. 6,024,513 discloses improvedmethods of decomposing municipal solid wastes placed in a landfill.

SUMMARY OF THE INVENTION

Biochemical decomposition of undesirable contaminants of gases,including nitrogen oxides, VOC's, carbon monoxide and sulfur oxides, isachieved by passing a gas stream through a managed landfill providingmicrobiological activity capable of degrading the contaminants. Gasessuitable for treatment include exhaust gases, landfill gas, odorousgases from putrefaction, and the like. In one preferred embodiment, thelandfill is a bioreactor type, where liquid is added to achieveincreased microbial action, and hence greater microbial activity indegrading both solid waste and gas contaminants. By permeating thepolluted gas through the landfill, there is a consumption of pollutinggases or vapors by microorganisms present in the landfill. The processenables increased combustion of fuels, such as landfill gas, that arecurrently wasted due to constraints on the pollutant emissionsassociated with their use, otherwise beneficially, as fuels for energyuses. In addition, the excess oxygen normally present in exhaust or fluegas can advantageously result in additional oxidative waste consumptionby microorganisms, yielding additional “air space” that is an economicbonus in extending landfill life and/or lessening landfill use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depicting the flow of gases into a landfillbiofiltration system.

FIG. 2 is a schematic of a landfill biofiltration system utilizingvertical wells for injection and withdrawal.

FIG. 3 is a schematic of a landfill biofiltration system utilizing thelandfill base layer for introduction of polluted gases, and, if desired,surficial layers for gas withdrawal.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention is drawn to methods and apparatus for removingpolluting gases, including nitrogen oxides (NO_(x)), carbon monoxide,sulfur oxides (SO_(x)) and volatile organic compounds VOC's from a gasstream, by passing the gas stream through a landfill biofiltrationsystem. Biologically mediated decomposition of these biodegradable gasestakes place in the landfill, thereby permitting the subsequent releaseof the cleansed gases.

The use of an entire landfill or sectors thereof for a gas pollutantbiofiltration system is advantageous over presently used biofilters forremediating or consuming gaseous pollutants. Presently used technologiesrequire a substantial increased incremental investment in equipment andmanagement that is not otherwise required. There are significant costsassociated with emplacement and fabrication of equipment for purposes ofbiofiltration. There are additional costs associated with themaintenance and management of the biofilter constructed or fabricatedsolely for the purpose of gas cleanup, including addition of liquid,nutrients and possibly cleaning of that material that supports thebacteria (“support”). The area requirement (“footprint”) and volumerequirement can also be inconveniently large. The present inventionprovides a solution to these problems by utilizing a resource that isreadily available, necessary in any event, and can be adapted so as toserve important functions as a biofilter, in addition to its normalwaste disposal role.

The landfill or waste dump has the advantage of enormous size relativeto any equipment that must ordinarily be constructed. This size providesa large surface area upon which the biological remediation reactions cantake place, and very long residence times compared to those available inother bioremediation equipment.

Before the present device and method for removal of gaseous contaminantsfrom polluted gases are disclosed and described, it is to be understoodthat this invention is not limited to the particular process steps andmaterials disclosed herein as such process steps and materials may varysomewhat. It is also to be understood that the terminology used hereinis used for the purpose of describing particular embodiments only and isnot intended to be limiting since the scope of the present inventionwill be limited only by the appended claims and functional equivalentsthereof.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include plural referents unless thecontext clearly dictates otherwise. Thus, for example, reference to “agas” includes a plurality of such gases and reference to “the landfill”includes reference to one or more landfills and equivalents thereofknown to those skilled in the art, and so forth.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood to one of ordinary skill inthe art to which this invention belongs. Although any methods, devicesand materials similar or equivalent to those described herein can beused in the practice or testing of the invention, the preferred methods,devices and materials are now described.

All publications mentioned herein are incorporated herein by referencefor the purpose of describing and disclosing, for example, the devicesand methodologies that are described in the publications which might beused in connection with the presently described invention. Thepublications discussed above and throughout the text are provided solelyfor their disclosure prior to the filing date of the presentapplication. Nothing herein is to be construed as an admission that theinventors are not entitled to antedate such disclosure by virtue ofprior invention.

Biofiltration. As used herein refers to the venting and permeation ofcontaminated air, vapors, or gases through a biologically activematerial comprising microorganisms capable of metabolizing one or moreof the polluting gases. A “biofilter” is a device containing abiologically active material or component through which contaminatedgases, vapors, or air are vented for reducing amounts or concentrationsof one or more contaminants or pollutants from the gases, vapors, orair. The biofilter of the present invention is a managed landfill, oralternately, waste dump of size comparable to a landfill. Such landfillsare typically large masses, usually at least about 1 ton in landfillmass, more usually at least about 100 tons of mass, and may be at leastabout 1000 tons or more in mass.

As used herein, “removing,” “removal,” and similar terms mean completelyor partially eliminating. For example, removing nitrogen oxidescomprises partial or complete biological reduction of the nitrogen inNO_(x) gases to molecular N₂ or other innocuously bound nitrogen asmight be fixed in protein or other compounds, thereby either reducing oreliminating NO_(x), concentrations. Gas means gas, vapor, or air.

Polluted gases. For the purposes of the invention, polluted orcontaminated gases contain one or more of the following biodegradablepolluting gases: nitrogen oxides, which include nitrous oxide (N₂O),nitric oxide (NO) and nitrogen dioxide (NO₂), generically referred to asNO_(x); carbon monoxide (CO); sulfur oxides, which include sulfurdioxide (SO₂) and sulfur trioxide (SO₃), generically referred to asSO_(x); volatile organic compounds (VOC's) and biodegradable noxioussmelling compounds such as from putrescing material, or other undesiredgaseous, biodegradable components. Adverse effects of many suchcompounds is such that emission standards are often set, especially inmore prosperous areas of the world. As stated above standards or limitsfor these compounds' emissions may be specific to such factors asprocesses, and locations such as US States, districts, or to countrieselsewhere.

Nitrogen oxides are a group of common air pollutants, occurring in theatmosphere as a result of both natural processes and processesassociated with human activities. Although there are eight recognizednitrogen oxides, three main gases, NO₂, N₂O and NO are of most concern.Two, NO and NO₂ are air pollutants contributing to the formation ofozone and photochemical reaction products, for which one common term is“smog”. Nitrous oxide, although not regarded by regulators as a localair pollutant per se, is nonetheless an extremely potent climate-active“greenhouse” gas of substantial concern as its atmosphericconcentrations continue rising worldwide.

Sulfur oxides are also quite common. Sulfur dioxide (SO₂) is the mostabundant. Sulfur oxides primarily result from combustion of coal andpetroleum, with the concentration of sulfur oxides in combustor productgases depending on the initial sulfur content. Like nitrogen oxides,they can have a direct harmful effect once in the atmosphere, as theyare toxic to biota and a respiratory irritant to humans. Sulfur oxidescan also have a secondary, or indirect harmful effect, by undergoing acommon reaction with water in the form of rain or fog to form dilutesulfuric acid, H₂SO₄, which along with other acidic compounds often inrainwater, comprises what is termed “acid rain”.

A variety of gases are found to be contaminated with these pollutantsand are suitable for cleansing by the methods of the invention. Suchcontaminated gases can result from combustion processes, e.g. exhaustfrom oil, coal, natural gas, etc. combustion; where the combustion maytake place at a factory site, electrical generating site, and the like.Contaminated gases can also result from initial landfill biodegradationprocesses, where contaminated landfill gas or aeration air is emitted,and also solid waste composting processes which have given rise tonumerous odor complaints across the US as reported in the tradeliterature.

Energies available from pollutant consumption are favorable for thebacteria which carry out pollutant consumption. The pollutant consumingreactions can take place in a variety of reactors, provided there issufficient aqueous phase containing nutrients, whether nutrients aresimple or complex. Reactions by which the desired bioremediation willtake place are known in microbiology and the art. Using glucose as thesurrogate for the monomer of the cellulose widely present in wastes,some examples of the important reactions and reaction energetics are asfollows: Nitrous oxide, N₂O 12N₂O + C₆H₁₂0₆ → 6CO₂ + 6H₂O + 12N2 ΔG_(o)= 81 kcal/mol O₂ consumed Nitric oxide, NO 12NO + C₆H₁₂0₆ → 6CO₂ +6H₂O + 6N₂ ΔG_(o) = 77 kcal/mol O₂ consumed Nitrogen dioxide, NO₂ 6NO₂ +C₆H₁₂0₆ → 6C0₂ + 6H₂0 + 4N2 ΔG_(o) = 62.4 kcal/mol O₂ consumed ammonia,NH₃ 4NH₃ + 3O₂ → 2N₂ + 6H₂O ΔG_(o) = 35.3 kcal/mol O₂ consumed carbonmonoxide, CO 2CO + O₂ → 2CO₂ ΔG_(o) = 61.5 kcal/mol O₂ consumedOxidizing gaseous propane C₃H₈ + 5O₂ → 3CO₂ + 4H₂O (representing ahydrocarbon ΔG_(o) = 47.6 kcal/mol O₂ consumed pollutant)

In general, the Gibbs free energy yield ΔG_(o) of these reactions isquite high, ranging from 35 to over 80 kilocalories per gram-atom ofoxygen reductively consumed This is whether in reducing oxygenchemically bound in NO_(x) or used in molecular (gaseous O₂) form inoxidizing carbon monoxide, ammonia or hydrocarbons. High free energy isa distinct metabolic advantage for the bacteria in carrying out theirmicrobiological pollutant degradation.

Degradation of sulfur dioxide may occur through the reaction sequence(where standard chemical nomenclature holds and M is a one cationequivalent such as sodium)2SO₂+2H₂O→2H₂SO₃H₂SO₃+2M⁺→2M⁺+SO₃ ⁼and, lastly2M⁺SO₃ ⁼ +½O ₂→M+2SO₄

Pertinently, the last “sulfite oxidation reaction” oxidizes sulfurdioxide and absorbs oxygen with sufficient rapidity that sulfitesolutions' oxygen uptake is used to simulate the oxygen uptake ofmicrobial systems (Augenstein, D. 1967 Oxygen Transfer in Fermentors atHigh Power Inputs. M. S. Thesis, Biochemical Engineering, Course XX,Massachusetts Institute of Technology Cambridge Mass.). The sulfate canalso undergo further reactions, but in the end these effectively resultin sequestration of the sulfur and preventing emissions of eitherhydrogen sulfide or sulfur oxide species.

Where the input contaminated gas is exhaust gas, it may require coolingprior to its introduction into the landfill serving as biofilter. Itwill be evident that in the specific instance of fuel combustor productgas, gas may exit processes at high temperatures, for some exhausts wellover 1000° F. Such temperatures are far in excess of the tolerance ofthe microorganisms likely to be active in consuming pollutants. Thus,for most combustor product gas, precooling will be required to suitablycool temperatures before introduction into any biofilter. A variety ofwell-documented cooling approaches can reduce the temperatures of thefuel combustor gases as necessary. For example heat exchangers or theducting of fuel combustor gases through a spray of water will work. Mostconveniently, highest fractional energy use of the fuel combustor gasremoves much to most of its sensible heat. As an example, theCaterpillar Co. sells combined heated power (CHP) engine-generator setswhich realize added sensible heat from the exhaust gas for a variety ofpurposes such as drying and other process heat (Caterpillar Co.)

Landfill biofiltration sections. Landfills for biofiltration may be ofconventional design with minor modifications. Alternately, bioreactorlandfills may be preferred. In an anaerobic landfill bioreactor, wastemoisture is increased, within constraints of extant federal or otherregulations, to speed completion of methane generation and methanerecovery to maximum potential. The supplementation may be combined withrecirculation of the exit liquid to achieve better distribution andcontacting. The relevance of anaerobic operation is that reducingreaction conditions that facilitate methane generation will alsofacilitate bacterial reduction of NO_(x), consuming its oxygen andleaving only harmless molecular nitrogen, nitrogen containingproteinaceous material, etc.

In an aerobic landfill bioreactor, air as well as water are introducedinto wet waste. One goal of an aerobic bioreactor is to consume organicwaste solids by oxidation to CO₂ and H₂O. In practice this generallyoccurs in parallel with some anaerobic activity, i.e. decomposition ofsome of the waste mass to methane and CO₂. In the developing aerobiclandfill technology, the consumption by oxidation of waste proceedsreadily with the addition of water and air (for example see Baker andJohnson (1999) Operational Characteristics and Enhanced Bioreduction ofMunicipal waste Landfill Mass by a Controlled Aerobic Process.Proceedings, 4th Annual Landfill Symposium, Solid Waste Association ofNorth America, Silver Spring, Md.). A representative reaction forglucose from waste cellulose is C₆H₁₂O₆+6O₂→6CO₂+6H₂O, ΔG_(o)=675.6 kcalor 56 kcal/gram-atom oxygen used. In contrast, the major reaction toproduce landfill gas, in wastes yields much less energy:C₆H₁₂O₆→CH₄+CO₂ΔG_(o)≈33 kcal/mol CH₄ produced.

Even where the energy yield is low, for example in the generation ofmethane, it is well established to occur in virtually every solid wastelandfill tested. Observed landfill gas production clearly indicatespresence of conditions permitting biological reactions within nearly allthose landfills wherein “landfill gas” is found and produced.Furthermore, the reactions mediating this decomposition of waste, andoxidative biological reactions in general, can be accelerated within thelandfill by a factor of at least ten by infiltrating and controllingmoisture and operating the landfill as a “bioreactor” (see Augenstein etal. 1998, and Augenstein et al. 2000, supra.)

Methods of Landfill Biofiltration

Landfills potentially provide enormous reaction volume for the desiredbiofiltration of exhaust gas and other noxious or undesirablecomponents. Landfills also, by nature of typical municipal solid wastes,provide great area upon the surface of the waste per unit volume of thelandfill. For example, the surface-to-external volume ratio of a 100micron diameter paper fiber, or lignaceous remnants of wood pulp such asnewsprint, representing examples of waste, and assuming 25-50% voids insitu, will be the order of about 200-400 cm²/cm³. The surface of wastedisposed in a typical landfill can function as a support for thebacteria that mediate desirable pollutant-consuming andcontaminant-consuming reactions. Further, landfills already exist, andtherefore the cost of setting up a bioreactor need not be incurred.

Landfills therefore have a large capacity for biofiltration. Forexample, depending on carburetion fuel/air ratio and other factors, aroughly 140,000 ft³/hr volume of exhaust gas may be generated from 1 MWeof electricity generation, which, (using terms often applied to water)is less than 4 “acre-feet of gas”. A 10-acre landfill, 60 feet deep, and(for example) 25% voids, could make available as much as 40 hoursdetention time for the exhaust gas from a 1 Megawatt electric (MWe)engine. And generally, there is even more landfill volume than thisreadily available at energy use sites.

The technology of bioreactors demonstrates attainability withinlandfills of circumstances needed to carry out the pollutantbiodegradation reactions above. A landfill with proper management ofliquid introduction and gas flow will quickly develop high microbialactivity necessary for the aerobic degradation of waste. Oxygen isnormally present in the fuel combustor exhaust gas at levels well inexcess of the microbe's requirement for oxidizing carbon monoxide andVOC's. It will be recognized that the oxidizing landfill waste itselfwill compete with the VOC, CO and sulfur oxide pollutants for theavailable oxygen. Where an organic waste remains available to beconsumed by oxygen in an aerobic landfill or sectional volume oflandfill, competition for oxygen may reduce oxygen in the gas so as tominimize pollutant remediation in the presence of waste oxidation.Competition for oxygen between waste and gaseous pollutant can beminimized by operating the landfill or landfill sector anaerobicallyuntil the landfills' oxygen consumption capability can be substantiallyreduced, and then using that sector of landfill to aerobically degradepollutants. Alternately, biologically inert components of the landfillsuch as pea gravel, chip tire, etc., can advantageously serve as zonesof oxidative pollutant remediation.

Given this consideration of competition between waste and pollutant foroxygen, the point of initial combustion product gas entry is preferablyat a locus in the landfill which has been well oxidized, with organicfractions reducing capacity limited to levels and rates such that oxygenremains available for the desired conversion and consumption of anyVOC's, carbon monoxide and sulfur oxides present in polluted gasintroduced into the landfill. The gas emitted from the combustionprocess may lack sufficient oxygen to allow the desired fractionalconversion of VOC's carbon monoxide, sulfur oxides, or other pollutantsubject to removal by oxidation. This would likely be the case for acombustor having little stoichiometric excess of air. In such systemssupplemental air may be added to the stream to be biofiltered.

In an anaerobic bioreactor, reducing conditions are optimized. A mixedmicrobial flora is capable of generating both intermediates fromcellulose, and ultimately methane from the waste (Augenstein, et al.1998 Yolo County Controlled Landfill Project. Proceedings, Symposium onLandfill Gas Assessment and Management, Ontario, Calif. April availablefrom the California Integrated Waste Management Board, Sacramento;Augenstein et al. (2000), supra.) will contain organisms well-suited forreducing nitrogenous oxides. Organic waste fractions, particularlylignin, remain after initial cellulose degradation, and are expected tocomprise 20%-50% of the organics after methane generation has completed.An examination of the stoichiometry reveals that such lignin and otherremnant organic wastes have a substantial capacity for the reductionreactions necessary to assimilate the nitrogen oxides.

All of the reactions described above give off heat (enthalpy) ofmagnitude rather similar to the free energies listed above. Themanagement of heat dissipation is integral to the use of landfillbioreactors. Heat generated by oxidative processes within a bioreactorare of such magnitude that its dissipation may take decades to centuriesto be lost from the waste mass if heat loss is by conduction alone(Augenstein (2000b) Bioreactor Landfills—some EngineeringConsiderations, presented and distributed at Wastecon 2000, Cincinnati,October, Available from SWANA, Silver Spring, Md.). This will hold ifoxygen forms over 1% of the polluted gas (including supplemental air)that is to be treated and there is sufficient reducing capacity such aslignin in the waste to consume the oxygen. This heat is most practicallyremoved by adding, or assuring the presence within waste, ofsupplemental water or aqueous liquid and using the latent heat of waterevaporating into the gas stream being treated. The amount of water thatmust be evaporated for the biofiltration process to function and not“cook to a stop”, is about 1.8 grams per kilocalorie generated withinthe waste or alternately expressed as about 1 pound of water evaporatedper 1000 Btu generated by microbial reactions within the waste.

Because the heat of oxidation of most organic compounds may lie betweenabout 35,000 and 85,000 kilogram-calories per kilogram-atom of oxygenconsumed in the reaction (as noted above), and conductive losses of heatare essentially negligible on the time scale of pollutant remediationreactions under consideration, the evaporative water consumption can beestimated as ranging between 30 60 and 100 200 grams per gram-atom ofoxygen consumed ancillary to the gas pollutant biofiltration reaction.The injection of gases and injection or presence of water in thelandfill is preferably at a level such as to achieve a temperature offrom about 70° to about 180° F. (about 21° to about 82° C.) in thelandfill cell. Temperature may be raised (within limits) by increasingair-to-liquid ratio. Temperature may be reduced by reducing the sameair-to liquid ratio. However sufficient contacting must also be assuredso that fractional oxygen absorption is high; contacting is facilitatedby having a large bacterial support surface to volume ratio and longresidence time as available in a landfill.

An integral part of the invention is the maintenance of proper moisturelevels, neither so low as to limit microbial activity nor so high as to“blind” and shut off pore spaces by filling them with water. Thus, thelandfill is preferably maintained at moisture levels between about 20%and about 60% moisture as a weight percentage of wet waste. Based on thewater sorption isotherm of cellulose, which can sorb over 5% of its ownweight of water in the solid phase, and other considerations includingthat food masses, mostly moisture, may comprise up to 15% of wet “blobs”relatively ineffective in biofiltration within waste, about 20% moisturelevel assures that at least some minimal moisture remains as aqueousphase spread over the waste which is acting as support. Values overabout 60%, may lead to excessive liquid flows and possible violation offederal constraints on head in the drainage layer over the base liner.One method of attaining a desirable moisture level is by “titrating”waste with water so that water just starts to drain from the bottom ofthe waste (Augenstein et al., (1998), supra.) Thereafter, makeup watershould be added in amount and on schedule as needed to make up forevaporative losses. The pH is maintained sufficient to facilitatefurther microbiological reactions. The pH of liquids withinwell-maintained anaerobic and aerobic landfills may be maintained in themost desirable ranges of 6 to 8.5 by adding acid or base or buffers asnecessary.

Waste will contain an abundance of organisms including those capable ofmetabolizing nearly all or all the “usual” solid waste components. Instartup, however, the organisms necessary for consuming pollutantsidentified above could likely to be present in fairly small numbers inthe waste. This is because waste does not normally contain thesepollutants. In the moist waste, exposure to gaseous contaminantcompounds enables growth of necessary bacteria to levels that allowoxidation of these compounds. In startup, in order that the pollutantdegrading organisms increase as much as possible without undesirableemissions levels, the process generating the gaseous pollutants maybegin at a slow rate. If, for example, the biofiltration substrate is aninternal combustion engine's exhaust, the engine may be started at partload. As exit gas analyses begin to show desirable fractional removalsof pollutants, the magnitude of engine output, or other pollutantproducing process may be increased. It is worth noting that with atypical microbial doubling time of 4 hours, a desired increase inorganisms of, e.g. 10⁶ fold would take only about 80 hours. Once thewaste microbial culture has developed to the point enabling pollutantremoval, the experience with aerobic landfills suggests intermittent“start-stop” operation should be straightforward.

The degree of pollutant abatement will be of concern in many cases, bothto regulators and others. If the sole input of gas to the landfill is anair-fuel combustor exhaust, the degree of abatement can be assessed bymeasuring the normalized concentration of the pollutant of concernagainst atmospheric nitrogen in combustor exhaust using standard meansfor gas analysis (e.g., gas chromatography, and the like). If bothcombustor exhaust, and air to facilitate oxidative processes areintroduced into the waste, a material balance can be attained andcontaminant reduction can be assessed relative to a low-cost low-leveltracer such as helium or sulfur hexafluoride carefully metered into thecombustor exhaust. An integrated surface scan can give much of theneeded data; however maximum accuracy in determination of fractionalcontaminant abatement from exit gas composition is attained bycollecting or combining most or all exit gas in a single stream.

Methods for the transport of gases into a landfill are known in the art,and are readily adapted from the prior art transport of oxygen or air,to the transport of contaminated gases as taught by the presentinvention. The contaminated gases, as previously described, may be theproduct of combustion, e.g. exhaust from factories and energygeneration, or noxious or odorous gases as evolved during certain phasesof waste composting.

In general, modes of gas introduction and withdrawal shown effective foraerobic landfilling, as it is known to those skilled in the art, willalso serve for the oxidative pollutant consumption reactions of thepresent invention. For most effective pollutant removal, a “plug flow”residence time distribution in which gas elements take as close-to-equalamounts of time as possible to pass through above waste zones will bethe most effective. The residence time or residence time distribution(the time range of fastest to slowest passage of gas through thelandfill) preferably lies within limits such that the pollutants arebiologically abated to the desired degree with a single pass. Thelandfill design must be such as to duct gas with the desired residencetime distribution through the waste. This will not only require gasinputs and outputs as indicated, for example, in FIGS. 1-3 but also thedegree of containment of the waste margins (top, bottom and sides) toconfine gas flows to prevent exit of too much “fugitive” untreated gasor “short circuiting”. If porous solid waste base or cover layers canallow short circuits, these can be of high area/volume support materialalready commonly used to construct such layers in landfills, e.g. sand,pea gravel, etc., which can accomplish biofiltration.

The specific design of the apparatus for the injection of air and waterinto the landfill to promote the aerobic composting reaction will varywith the specific requirements of the landfill. The general design caninclude the venting of depleted air and water vapor at the top of thelandfill and the venting of depleted air, water vapor and leachate atthe bottom, e.g. with a series of perforated pipes in roughly planar andnear-level form conforming to the top and bottom of the landfill. Thepipes are spaced depending on the mass and void volume of the wastesection to be utilized for biofiltration. The piping system may bevalved so that different parts of the cell can be treated with greateror lesser flows in order to accommodate variations in bioremediationabilities of those different parts of the cells. The flow programming ofthe injection into the landfill can be based on the measurement oftemperature in the cell, and on the oxygen/carbon dioxide and anyremnant pollutant content of the off gas from the cell.

A grid of gas injection wells may be established throughout thelandfill. The landfill's gas injection system can comprise gas streamblowers or pumps that are connected to an existing leachate collectionsystem cleanout ports, which are typically located along the sides of alandfill. Areas that can filter and receive more contaminated gas as thesystem operates are augmented by additional contaminated gas supplythrough the vertical gas injection. The gas header or duct piping may becorrugated plastic agricultural drainage pipe if high flexibility andload resistance are needed. Vertical gas wells may be made of plastic(Polyvinyl chloride (PVC) being often used), or metal pipe set atappropriate intervals throughout the waste. Or, wells may simply be flowchannels of highly porous material (such as gravel or shred tires) ofhigh permeability relative to waste as described in Augenstein et. al.(1998), supra. As polluted gas is forced into inlet ports orintroduction surfaces, or the leachate collection system, and into thewaste, the gas distribution system is pressurized and managed so thatthe gas travels outwardly through the piping slotting (or screenedcasing) and upward through the waste mass with maximum possibleuniformity, that is, all elements of introduced gas spend times withinthe waste that are as similar as possible. Alternatively, vertical wellsconsisting of standard well component materials, and which may haveexternal casing, or screens, are installed vertically into the wastemass and connected together via a common header system (piping). Usingthe wells, pollutant-contaminated gas may be either introduced into thewaste or pulled through the waste from the gas's introduction pointselsewhere. Blowers connect to provide gas to the header system and tothe vertical wells, or vacuum from identical blowers may pull gas fromthe waste when it is introduced elsewhere into the header system andthence to exhaust. The important feature of any of the possiblearrangements for pollutant-contaminated gas introduction and withdrawalis that it assure at minimum a gas residence and waste contact time thatis adequate for required pollutant bioremediation.

An additional feature of the invention is that heats generated byreactions need to be dealt with so that highly exothermic energyyielding reactions, as described above, do not cause the bioremediationto “cook to a stop” or evaporate water in the waste to the point wherewaste is “dried to inactivity”. Calculations show that conduction ofheat away from the waste mass will in most cases be unacceptably slow.Practical and necessary heat loss rates are assured by presence oraddition, and subsequent evaporation into the gas stream of sufficientmoisture so that the liquid's or aqueous amendment's latent heat ofevaporation carries away the heat generated by oxidative biological orother exothermic reactions occurring in the waste. The wells forintroducing polluted-gas or multiple purpose gas addition/extractionwells are installed into waste and operated, and moisture assured added,via approaches as well-known in the art. Examples of such moistureaddition methods are described (Augenstein et al. (1998), supra.; Paceyet al. (1999), supra.) Normal, natural precipitation may also comprise apart of the water component. Aqueous amendments serve to provide thenecessary supplemental moisture to evaporatively dissipate heatgenerated by oxidative reactions in the landfill. Other describedmoisture addition methods will work providing distribution is achieved.

The gas flow rate and liquid flow rates are adjusted by either adjustingor throttling gas and liquid wells, selectively isolating gas or liquidwells through valving, thereby shutting off or increasing gas or liquidto selected areas as needed. One very useful option is to follow, adjustand assure moisture distribution through use of in-waste moisture andtemperature sensors (Augenstein et al., (1998), supra.) The moisturesensors have been demonstrated to show clearly the key moisture profilein the waste. Temperature sensors provide an indication of the reactionrates and exotherm in vicinity of the sensors. Temperature-humidityreadings of gas slightly beneath or at the surface of the waste willalso indicate levels of desired biological activity in the wasteelements beneath the zone where readings are taken. Another operationaloption, where gas and particularly oxygen transfer may be arate-limiting step, is adjustment (lengthening) of polluted gasresidence time to allow sufficient oxygen consumption, thence heatgeneration, and thence water vapor partial pressure and evaporation sothat all oxygen is consumed and anaerobiosis, where necessary, isachieved. Finally, tracers may be used as an aid in determining reactionprogress and material balance as discussed later.

FIG. 1 shows schematically one embodiment of the invention. The fuel inFIG. 1 is “landfill gas” extracted from an appropriately activegas-generating section of a landfill. The landfill is comprised of awaste sector 30, and may further comprise one or more sectors ofdegraded waste 25. The anaerobic section of the landfill is shown to beshaded, and the aerobic section is unshaded. The landfill may besituated above a porous base layer 35, and be covered with suitablewaste cover layers 10, which may include a membrane cover 15. Apermeable gas extraction layer 20 may also be present. The fuelcombustor is an engine 1 operated on landfill gas 2 extracted from asector of the landfill which is still actively producing methane. Theengine exhaust gas is ducted through a cooling section 3 where gas iscooled by means well-known to the art, e.g. water spray, heat exchange,etc. to cool the gas, realize benefits of additional exhaust heat, andthe like. The gas is cooled to below 300° F., preferably down to its dewpoint to minimize (in the case of the normal excess-oxygen combustors)any danger of fire or pyrolytic product formation. Added introduced airflow may be needed to evaporatively dissipate heat, or allow fulloxidation and bioremediation of pollutants or both. In such cases air isadded to provide at least that necessary excess of saidoxygen-containing gas to allow oxidation of the CO, VOC's and SOx in thelandfill. Alternately, the necessary criterion may be that the airaddition is sufficient to allow the necessary evaporation to dissipatethe heats of reaction.

FIG. 2 shows one arrangement of wells demonstrated as workable for gas(oxygen from atmospheric air) transfer. Vertical wells 4 and 5 are usedfor introduction of contaminated gas 40 and extraction of clean gas 45,respectively. Any moisture essential to permit reaction is added throughinlet(s) represented by 6. The vertical wells have a perforated zone 70for flow of gas. Although only one gas introduction well 4 and one gaswithdrawal well 5 is shown in FIG. 2, there may be a multiplicity of gasintroduction and withdrawal wells operating in tandem so as toaccomplish the necessary gas contaminant bioremediation. The aerobiczone of the landfill 25 is depicted as unshaded 60, while the reducingzone (O₂ absent) is depicted as shaded 55. The base layers 35, which maybe gas-conducting, and gas-conducting cover may also be of substratessuch as sand or gravel upon which bioremediation can take place. Thewhole of the landfill area wherein bioremediation occurs may also havelow-permeability cover, which may be the common low permeability clays,or, for example, geomembrane above a porous surface layer. Liquid isadded as appropriate at locations at the surface (shown by 6 in FIG. 2)or within the waste.

FIG. 3 shows an alternative arrangement, in which the fuel combustor orother contaminated product gas passes vertically upward through thewaste using a permeable base layer as a zone of gas introduction. As inFIGS. 1 and 2, the reducing zones 55 and 60 are shown by shading. Inthis embodiment, the input gas 40 is input through a gas inlet line 7and the clean, biofiltered gas 45 is then released to the atmosphere viaa permeable gas extraction layer 20. Wells 50 would normally be presentin a degraded landfill but are not required for the present invention,and are shown for completeness only.

As gas is ducted (pumped) or drawn into the aerobic and thence anaerobiczones of the landfill it can be expected, at the first startup, thatnecessary organisms may need to be allowed time to grow to necessarylevels. In the aerobic zone, the gas from the aerobic zone of thebioreactor will necessarily be slowly depleted of some amount of itsoxygen (O₂), by reaction with the pollutant materials (VOC's, CO, andthe like) and also by reaction of O₂ with remnant landfilled wasteorganics. Once oxidizable pollutants and oxygen are removed, normalprogress of the gas stream will be to an anaerobic zone, in whichreducing organics such as lignin remain. The conditions in the anaerobiczone (expected to be the majority of the landfill) favors theconsumption of the nitrogenous oxides.

“Channeling” or “short circuiting” may occur, such as to limitbioremediation of some fraction of gaseous pollutants. In thisphenomenon, gas might flow through preferred channels within waste,without necessary gas-to-waste contacting to achieve desiredbioremediation. For such conditions, channeling may be detected byintroducing with the contaminated gas, easily quantified non-reactivegaseous tracers not ordinarily present in the interstitial gas withinthe waste. One such gas is helium, and another, sulfur hexafluoride,however many nonreactive gases available, particularly noble gases, cansuffice. The value of these tracers is that timing or their emission andzones of their emission may be detected, as well as degrees ofremediation. When problems are detected, gas flow can be shut off toareas where gas channeling is occurring, with consequent redirectionthrough areas of active bioremediation.

1. A method for removing biodegradable gaseous pollutants fromcontaminated gases, the method comprising: permeating said contaminatedgases into a 1000 ton or more tonnage landfill waste mass, wherein saidgaseous pollutants comprise nitrogen oxides, including one or more ofnitrous oxide (N₂O), nitric oxide (NO) and nitrogen dioxide (NO₂); andmaintaining said waste mass such that microorganisms present in saidwaste mass biodegrade said gaseous pollutants to substantially reducesaid nitrogen oxides to N₂; wherein the contaminated gases are exhaustgases from combustion of landfill gas.
 2. The method according to claim1, wherein said gaseous pollutants further comprise one or more of:carbon monoxide (CO); sulfur oxides; and volatile organic compounds(VOC).
 3. The method according to claim 2, wherein said sulfur oxidescomprise one or more of sulfur dioxide (SO₂) and sulfur trioxide (SO₃).4. The method according to claim 1, wherein said exhaust gases arecooled to a temperature of less than about 150° C.
 5. The method ofclaim 1, wherein said waste mass comprises both aerobic and anaerobicsectors.
 6. The method according to claim 1 further comprising filteringsaid contaminated gases through a biofilter on inert supports to carryout a process of oxidative biofiltration.
 7. The method according toclaim 6, wherein said inert supports comprise organic and inorganicsize-reduced construction and demolition wastes.
 8. The method of claim1, in which one or more of waste cover, base materials, and formerlandfill gas wells are filled with non-waste material to serve forbiofiltration.
 9. The method of claim 1, in which zones, rates andpercentages of contaminant reduction by bioremediation are assessed andthence controlled by means relying on localization, at points in orabove the waste, of gas contaminant concentrations relative to knowninert gas or inert gas tracer inputs.
 10. The method of claim 1 in whichthe overall degree of biofiltration is assessed by contaminant reductionmeasurement relative to known inert gas or inert gas tracer inputs at asingle gas outlet location where all gas exiting the biofiltration iscommingled and well mixed.
 11. The method of claim 1 in which theoverall degree of contaminant emission in gas exiting the process isassessed from contaminant concentrations and flow of gas exiting thewaste mass.
 12. The method according to claim 1, further comprising:measuring oxygen consumption in the waste mass; and adding an aqueousamendment to the waste mass in an amount based on the oxygen consumed inthe waste mass.
 13. The method according to claim 12 wherein the aqueousamendment is added at a volume of from 50 to 120 ml water pergram-atomic weight of oxygen consumed in the waste mass.
 14. The methodaccording to claim 1, further comprising: measuring temperature in thewaste mass; and adding an aqueous amendment to the waste mass in anamount based on the temperature in the waste mass.
 15. The methodaccording to claim 2, wherein said gaseous pollutants comprise carbonmonoxide, wherein the microorganisms in said waste mass biodegrade saidgaseous pollutants to substantially convert said carbon monoxide tocarbon dioxide.
 16. The method of claim 1 wherein an aqueous amendmentis provided to said landfill waste mass to maintain an aqueous level ata volume of from 50 to 120 ml water per gram atomic weight of oxygenconsumed in a biodegradation reaction.
 17. A method for removingbiodegradable gaseous pollutants from contaminated gases, the methodcomprising: permeating said contaminated gases into a landfill of atleast 1000 tons in landfill mass; measuring oxygen consumption in saidlandfill; providing an aqueous amendment to said landfill at a volume offrom 50 to 120 ml water per gram atomic weight of oxygen consumed in thelandfill; and maintaining said landfill such that microorganisms presentin said landfill substantially reduce said gaseous contaminants; whereinthe contaminated gases are exhaust gases from combustion of landfillgas.
 18. The method of claim 17, wherein said aqueous amendmentcomprises buffers and nutrients.
 19. The method of claim 17 in whichsaid aqueous amendment is a wastewater.
 20. The method according toclaim 17, wherein said landfill comprises at least 40 hours detentiontime for said contaminated gases.
 21. The method according to claim 17,wherein said gaseous pollutants are reduced to less than 0.06 pounds ofNO_(x) per one million Btus and 0.2 pounds of carbon monoxide permillion Btus.